That Catholic decline when also considering that Catholic immigration is quite high provides more proof that very few people ever disidentify as Catholic whether practicing or not because "CULTURE!" is an outdated and now absurd notion.
When you look at the numbers for retention and conversion, the reason why the number of Catholics is shrinking is less about poor retention (the retention isn't much different than the other groups) but a lack of converting anyone.
Interestingly, the nones seem to have the opposite problem, a lot of converts but a surprisingly low retention rate.
